1. Technical Field of the Invention
The present invention relates generally to methods of manufacturing stators for dynamoelectric machines. More particularly, the invention relates to a method of manufacturing a stator for a dynamoelectric machine, which is characterized by a process of mounting a stator coil to a stator core that has a unitary structure.
2. Description of the Related Art
In recent years, dynamoelectric machines, such as electric motors and electric generators, have been required to be compact, be able to output high power, and have high quality.
In particular, for dynamoelectric machines for use in motor vehicles, the spaces available for installation of those machines in the motor vehicles have been decreasing, while the needs for them to output high power have been increasing.
Japanese Patent First Publication No. 2002-51485 discloses a stator for a dynamoelectric machine. The stator includes a stator core that is composed of a plurality of stator core pieces each having a coil wound thereon. The stator further includes a case for receiving the stator core pieces, which is composed of a pair of cylindrical inner and outer cases. The inner case temporarily holds the stator core pieces that are disposed in an annular form on the radially inner surface of the inner case. The outer case is shrinkage-fitted on the radially outer surface of the inner case, thereby fixing the stator core pieces in the inner case.
Japanese Patent No. 3982446, an English equivalent of which is US 2007/0180682 A1, discloses a method of manufacturing a stator for an electric rotating machine (or a dynamoelectric machine). According to the method, a stator winding set is formed to have a hollow cylindrical shape, disposed radially inside of a hollow cylindrical stator core, and expanded and inserted into slots formed in the radially inner surface of the stator core.
Moreover, there has been known a method of manufacturing a stator coil. According to the method, a plurality of shaped electric wires are first obtained by shaping a plurality of straight electric wires. Each of the shaped electric wires is so shaped as to include a plurality of straight portions parallel to each other and a plurality of connecting portions connecting the straight portions. The shaped electric wires are assembled together to make up a flat electric wire assembly.
In the flat electric wire assembly, a plurality of shaped electric wire pairs, each of which consists of two shaped electric wires, are arranged to extend in the longitudinal direction of the assembly in parallel with each other. Further, each of the shaped electric wire pairs includes a plurality of straight overlapped portions that are formed by overlapping the straight portions of one of the two shaped electric wires of the pair respectively with those of the other. The straight overlapped portions each extend perpendicular to the longitudinal direction of the flat electric wire assembly and are spaced in the longitudinal direction at predetermined intervals.
The flat electric wire assembly is further rolled around a cylindrical core member by a predetermined number of turns, thereby forming a hollow cylindrical electric wire assembly. The hollow cylindrical electric wire assembly includes a plurality of straight stacked portions, in each of which a predetermined number of the straight overlapped portions of the flat electric wire assembly are stacked together in the radial direction of the assembly. The straight stacked portions each extend parallel to the axial direction of the hollow cylindrical electric wire assembly, and are spaced at predetermined intervals in the circumferential direction of the same.
The hollow cylindrical electric wire assembly can make up a stator coil of a stator only after being mounted to a stator core such that: each of the straight stacked portions of the assembly is fitted in one of a plurality of slots of the stator core; and all of the connecting portions of the shaped electric wire pairs of the assembly are located outside the slots of the stator core.
However, when the stator core has a unitary (or one-piece) structure, it is very difficult to mount the hollow cylindrical electric wire assembly to the stator core.
To solve this problem, one may consider employing the divided stator core structure as disclosed in Japanese Patent First Publication No. 2002-51485. More specifically, the stator core might be composed of a plurality of stator core pieces. Further, the stator core pieces might be separately fitted onto the hollow cylindrical electric wire assembly from the radially outer side. Then, a hollow cylindrical case might be fitted onto the radially outer surface of the stator core, thereby holding the stator core pieces along with the hollow cylindrical electric wire assembly (i.e., the stator coil).
However, with the divided stator core structure, it would be difficult to realize a suitable magnetic circuit of the stator.
Therefore, there is a strong demand for a manufacturing method by which the hollow cylindrical electric wire assembly can be easily mounted to the stator core which has a unitary structure.
Moreover, according to the method disclosed in Japanese Patent No. 3982446, the stator winding set is inserted into the slots of the stator core only by pressing the coil end parts of the stator winding set. Therefore, it is necessary to apply large compression forces to the coil end parts. However, this will cause the coil end parts to be greatly deformed, thereby increasing the air gaps between the shaped electric wires making up the stator winding set. Moreover, to allow application of large pressing forces to the coil end parts, it is necessary to set large axial heights of the coil end parts. However, with the large axial heights of the coil end parts, the axial dimension of the stator will be increased and the performance of the stator will be lowered. In addition, the stator winding set disclosed in Japanese Patent No. 3982446 is different in structure from the hollow cylindrical electric wire assembly described above.